Mavs get a great off-guard to play next to Kyrie; his functional athleticism is much needed for this backcourt. He fits beautifully next to Flagg and AD, giving them a guy who can spot up or attack off the catch and make the most of advantage situations. Losing PJ hurts, but the long-term hole at SG is sizeable right now (Klay is great, but getting older) and the PF spot is already stacked for the Mavs (AD, Flagg and Naji will have reps there)

Spurs get a A+++ shooter in Klay to help stretch the floor for Harper/Fox/Wemby etc and an incredibly versatile defender/creator in PJ at arguably their weakest position on the roster. Not to mention Klay is an incredible vet to have in the locker room, which is valueable since CP3 is likely out the door. Klay's contract will be up in 2 years, giving the Spurs extra cap flexibility (which they will need when all of their young guys get extended). PJ would likely extend for something in the range of 20 mil/year, which would be insane value for what he brings to the table; an All-NBA level defender and excellent rebounder who can do also do a bit of everything offensively (dribble, pass, shoot, post-up).

Vassel is a nice piece for SA but he's been starting games at the 3, out of position as hes naturally more of a 2, size-wise. Having drafted Carter-Bryant, Barnes/Keldon still on the roster and a log-jam of quality guards (Fox, Harper, and Castle all deserve 30+ minutes a night), Vassel is expendable. I think Klay could easily step into the medium-sized role that Vassel would have this upcoming year and excel in it.

post trade healthy lineups:

Mavs: Kyrie, Vassel, Flagg, AD, Lively Spurs: Fox, Harper, Carter Bryant, PJ, Wemby

WIN-WIN